A light source apparatus used for a projector apparatus is described in Japanese Patent Application Publication Nos. 2007-087637, 2003-347071, 2002-015883 and 2007-165067. Japanese Patent Application Publication No. 2007-087637 discloses a light source apparatus having a lamp lighted with alternating current, which is used for a projector apparatus, wherein it is possible to prevent formation of an unnecessary projection(s) by periodically inserting a low frequency into a steady frequency (refer to paragraph [0021] of the patent application publication). Japanese Patent Application Publication No. 2003-347071 discloses a light source apparatus, used for a projector apparatus having a lamp, which is lighted with alternating current. Japanese
Patent Application Publication No. 2003-347071 discloses that a lamp is vertically arranged and lighted, wherein when the vertically arranged lamp is lighted, time T1, during which voltage is applied to an upper electrode serving as a negative electrode, is longer than time T2, during which voltage is applied to a lower electrode serving as a negative electrode, whereby it is possible to suppress a rise in temperature of the upper electrode (for example, refer to paragraph [0029] of the patent application publication).
Japanese Patent Application Publication No. 2002-015883 discloses a gas discharge lamp for a video projector, which is lighted with alternating current, wherein two or more operation frequencies, for example, 45, 65, 90, and130 Hz, are used to form a projection at an electrode tip. Moreover, when alternating current is inputted, a current value is changed in pulse form (refer to FIG. 1 of Japanese Patent Application Publication No. 2007-165067). Japanese Patent Application Publication No. 2007-165067 discloses a light source apparatus having a lamp lighted with alternating current, which is used for a projector apparatus, wherein a color wheel is used for the projector apparatus (for example, refer to paragraph [0013] of the patent application publication).
A projector apparatus may be used for advertising media with an image called digital signage, wherein such media are required to be displayed in various directions or at various places because of the nature of advertisement. Usually, such a light source apparatus for digital signage is not set in a fixed projecting direction or a fixed projection place, that is, a lamp is sometimes required to be horizontally placed to light the lamp, or sometimes vertically placed to light the lamp. Thus, such a light source apparatus for digital signage is expected so that the lamp, which is provided in the light source apparatus, can be lighted even if it is placed either horizontally or vertically. A light source apparatus disclosed in Japanese Patent Application Publication No. 2007-087637 is designed so that the lamp is horizontally arranged. When such a lamp is horizontally arranged, a duty ratio of current supplied to the lamp is generally approximately 1:1. In such a light source apparatus disclosed in Japanese Patent Application Publication No. 2007-087637, if the lamp is lighted when the lamp is vertically arranged, a heat convection arises inside the lamp so that the temperature of the upper electrode becomes higher than that of the lower electrode. In such a kind of lamp, since the electrode is overheated, and in addition to the overheating, heating due to the heat convention is added, there is a problem that the upper electrode melts and is damaged. Moreover, even if the lower electrode cools down more than the upper electrode, so that even if low frequency is inserted, formation of an unnecessary projection cannot be suppressed.
The light source apparatus, which is disclosed in Japanese Patent Application Publication No. 2003-347071, is designed so that a lamp is vertically arranged, and a duty ratio of current supplied to the lamp is different. Thus, when the lamp is horizontally arranged in such a light source apparatus, there is a problem that one of electrodes is overheated, more than the other electrode, so that the one of the electrodes may be damaged. As in the apparatus disclosed in Japanese Patent Application Publication No. 2007-087637, in a light source apparatus disclosed in Japanese Patent Application Publication No. 2002-015883, a lamp used in a vertical arrangement is not assumed. Moreover, in some of such light source apparatuses used for a projector apparatus, as shown in Japanese Patent Application Publication No. 2007-165067, light is emitted through a color wheel, which is divided into R, G, B, and W areas. However, when such a color wheel is used, if the polarity of current to be supplied to the lamp changes in a portion between two adjacent areas of the R, G, B, and W, a ripple arises, so that the illumination of light from the lamp becomes temporarily high and low (bright and dark). Therefore, as shown in FIG. 13A, it is desirable to match area change timing of the R, G, B, and W areas of such a color wheel with polarity change timing. In the light source apparatus disclosed in Japanese Patent Application Publication No. 2003-347071, the lamp is designed to be arranged vertically, and a duty ratio of current supplied to a lamp (ratio of a period, in which the polarity of the current is positive, to a negative period thereof) is not set to 1:1, so that, as shown in FIG. 13B, when it is applied to an apparatus, in which a color wheel is used, the current polarity change timing and area change timing of the R, G, B, and W areas of the color wheel are not necessarily in agreement with each other. Therefore, due to a ripple, which occurs when the current polarity changes, the illumination of light from the lamp becomes temporarily high and low (bright and dark). Thus, flickering occurs.
Moreover, although the entire liquid crystal screen on a display is refreshed at a fixed rate (refresh rate) on the display, if this refresh rate (vertical frequency) and the polarity change timing of the current impressed to the electrode of the lamp are not synchronized flickering occurs, as in the case of the above-mentioned color wheel. In the case of Japanese Patent Application Publication No. 2003-347071, as shown in FIG. 13B, since the duty ratio of current does not necessarily turn into 1:1, timing of refreshing the entire liquid crystal screen and current polarity change timing are not in agreement. Thus, similarly to the color wheel case, the illumination of light from the lamp becomes temporarily high and low (bright and dark) and flickering occurs.
As mentioned above, the light source apparatus of the prior art is not configured so that the lamp can be lighted in either a horizontal arrangement or a vertical arrangement, and when a light source apparatus designed so that the lamp thereof is lighted in a horizontal arrangement, is installed and lighted in a vertical arrangement, there is a problem, on which an upper electrode melted and damaged. Moreover, as disclosed in Japanese Patent Application Publication No. 2003-347071, it is proposed that such a lamp can be used in vertical arrangement, by setting a period, in which the polarity of current supplied to the lamp is positive, so as to be different from a negative current period. However, when a color wheel or a liquid crystal display is used, current polarity change timing is not necessarily in agreement with the change timing of the R, G, B and W areas of the color wheel, or the refreshment timing of a liquid crystal display, so that there is a problem on which a flicker occurs on the screen.